The invention relates to a structured body for an anode used in fuel cells which is briefly termed an anode structure in the following. The invention also relates to a high temperature fuel cell having such an anode structure and to a method of manufacturing the anode structure in accordance with the invention.
A high temperature fuel cell (SOFC fuel cell) is known from DE-A-19 819 453 in which an anode substrate forms a support structure. An intermediate anode layer, a solid electrolyte, preferably in the form of a very thin layer, and a layer-like cathode are applied to this support structure. The anode substrate and the intermediate anode layer are manufactured from the same material, namely from a porous cermet which consists of the ceramic material YSZ (yttrium stabilized zirconium oxide), which is also used as the solid electrolyte, and nickel. Electro-chemical reactions take place at so-called three-phase points (nickel/electrolyte/pore) in the contact region between the anode and the electrolyte in which nickel atoms are oxidized by oxygen ions (O2−) of the electrolyte and these are reduced again by a gaseous fuel (H2, CO), with H2O and CO2 being formed and the electrons released in the oxidization being passed on by the anode substrate. To obtain a large density of points for these three-phase points, a composition of the intermediate anode layer is provided for which the ratio of the proportions by volume of nickel, YSZ and pores lies close to 1:1:1.
The aim of the largest possible density of the three-phase points is, however, of lesser importance with respect to a further problem, namely with respect to the requirement that the anode should have a so-called redox stability. The redox stability relates to the properties of the electrode material with respect to a multiple change between oxidizing and reducing conditions. On the one hand, this change, which is briefly termed a redox change in the following, should not result in any major changes in properties for the ceramic components. On the other hand, an irreversible change, i.e. an ageing of the metallic components as a result of the redox change, should be influenced by means of the constant ceramic components such that the electrical conductivity of the electrode material is largely maintained. With such an ageing, a grain growth of the nickel takes place in which large crystallites grow at the cost of small ones and so allow gaps to arise in electrically conductive connections of the anode structure.
The redox stability is very important in practice because, according to experience, it is not possible to keep a battery with fuel cells in continuous operation. At each operation stop, the supply of the fuel must be stopped for safety reasons. When the gaseous fuel is absent, oxygen penetrates onto the anode sides of the fuel cells and the previously reduced state of the nickel changes into the oxidized state. When fuel cells are used in domestic technology for the purpose of the simultaneous production of electrical and thermal energy, around 20 interruptions to operation can be expected per year. A fuel cell must be usable for around five years for economical reasons. The fuel cell must thus only age so fast that up to 100 redox changes are possible.
However, in addition to the redox stability, good gas permeability of the anode structure is also important, as is—with respect to commercial use—a production of the anode structure which should be economical.